New SASS guide

More than happy to hear about...

* Stuff I've missed
* Bad examples
* Poor English
* Problems with structure
* My awesomeness :?:
* ...
* Just plain questions

I expect to follow this up as the component library/styleguide becomes clearer. And as we roll out some KSS into the new Rails happy path.

Expect a presentation version of this too in short(ish) order.
It is open. Seems to be common practice, and doesn't hold any deep secrets. As we start fleshing it out, we should be proud of it.

If it is strongly objected to being 'in public' I can do a quick PR to shut it down.